Refactor issue dedup system to use claude-code-action with /dedupe command

Replace the copilot-generated inline search logic with a claude-code-action
based architecture inspired by anthropic/claude-code's approach:

- Add .claude/commands/dedupe.md with 5-parallel-search strategy
- Add scripts/comment-on-duplicates.sh with 3-day grace period warning
- Rewrite issue-dedupe.yml to use claude-code-action + /dedupe command
- Rewrite autoclose script to check bot comments, human activity, and thumbsdown
- Rewrite backfill to trigger dedupe workflow per issue with rate limiting
- Add concurrency control, timeout, input validation, and rate limit retry
- Remove gh.sh (unnecessary), backfill-dedupe.js (replaced by workflow trigger)

scripts/comment-on-duplicates.sh

@@ -0,0 +1,106 @@
+#!/usr/bin/env bash
+#
+# comment-on-duplicates.sh - Posts a duplicate issue comment with auto-close warning.
+#
+# Usage:
+#   ./scripts/comment-on-duplicates.sh --base-issue 123 --potential-duplicates 456 789
+#
+set -euo pipefail
+
+REPO="${GITHUB_REPOSITORY:-}"
+if [ -z "$REPO" ]; then
+  echo "Error: GITHUB_REPOSITORY is not set" >&2
+  exit 1
+fi
+
+BASE_ISSUE=""
+DUPLICATES=()
+
+# Parse arguments
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --base-issue)
+      BASE_ISSUE="$2"
+      shift 2
+      ;;
+    --potential-duplicates)
+      shift
+      while [[ $# -gt 0 && ! "$1" =~ ^-- ]]; do
+        DUPLICATES+=("$1")
+        shift
+      done
+      ;;
+    *)
+      echo "Error: Unknown argument: $1" >&2
+      exit 1
+      ;;
+  esac
+done
+
+# Validate inputs
+if [ -z "$BASE_ISSUE" ]; then
+  echo "Error: --base-issue is required" >&2
+  exit 1
+fi
+
+if ! [[ "$BASE_ISSUE" =~ ^[0-9]+$ ]]; then
+  echo "Error: --base-issue must be a number, got: $BASE_ISSUE" >&2
+  exit 1
+fi
+
+if [ ${#DUPLICATES[@]} -eq 0 ]; then
+  echo "Error: --potential-duplicates requires at least one issue number" >&2
+  exit 1
+fi
+
+for dup in "${DUPLICATES[@]}"; do
+  if ! [[ "$dup" =~ ^[0-9]+$ ]]; then
+    echo "Error: duplicate issue must be a number, got: $dup" >&2
+    exit 1
+  fi
+done
+
+# Limit to 3 duplicates max
+if [ ${#DUPLICATES[@]} -gt 3 ]; then
+  echo "Warning: Limiting to first 3 duplicates" >&2
+  DUPLICATES=("${DUPLICATES[@]:0:3}")
+fi
+
+# Validate that the base issue exists and is open
+if ! gh issue view "$BASE_ISSUE" --repo "$REPO" --json state -q '.state' | grep -qi 'open'; then
+  echo "Error: Issue #$BASE_ISSUE is not open or does not exist" >&2
+  exit 1
+fi
+
+# Build the duplicate links list
+LINKS=""
+COUNT=0
+for dup in "${DUPLICATES[@]}"; do
+  # Validate duplicate issue exists
+  if gh issue view "$dup" --repo "$REPO" --json number -q '.number' > /dev/null 2>&1; then
+    COUNT=$((COUNT + 1))
+    LINKS="${LINKS}${COUNT}. https://github.com/${REPO}/issues/${dup}
+"
+  else
+    echo "Warning: Issue #$dup does not exist, skipping" >&2
+  fi
+done
+
+if [ "$COUNT" -eq 0 ]; then
+  echo "Error: None of the specified duplicate issues exist" >&2
+  exit 1
+fi
+
+# Build and post the comment
+COMMENT="Found ${COUNT} possible duplicate issue(s):
+
+${LINKS}
+This issue will be automatically closed as a duplicate in 3 days.
+- To prevent auto-closure, add a comment or react with :thumbsdown: on this comment."
+
+gh issue comment "$BASE_ISSUE" --repo "$REPO" --body "$COMMENT"
+
+# Add the duplicate label
+gh issue edit "$BASE_ISSUE" --repo "$REPO" --add-label "duplicate"
+
+echo "Posted duplicate comment on issue #$BASE_ISSUE with $COUNT potential duplicate(s)"